Methods and apparatus for improving micro-LED devices

ABSTRACT

A μLED device comprising: a substrate and an epitaxial layer grown on the substrate and comprising a semiconductor material, wherein at least a portion of the substrate and the epitaxial layer define a mesa; an active layer within the mesa and configured, on application of an electrical current, to generate light for emission through a light emitting surface of the substrate opposite the mesa, wherein the crystal lattice structure of the substrate and the epitaxial layer is arranged such that a c-plane of the crystal lattice structure is misaligned with respect to the light emitting surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to United Kingdom Patent ApplicationNo. 1408084.0 filed May 7, 2014, which is hereby incorporated in itsentirety.

TECHNICAL FIELD

The invention relates to micro-LEDs (μLEDs). More specifically, theinvention relates to methods and apparatus for guiding light emittedfrom μLEDs in order to improve or alter their performance.

BACKGROUND

LEDs convert electrical energy into optical energy. In semiconductorLEDs, light is usually generated through recombination of electrons,originating from an n-type doped semiconductor layer, and holesoriginating from a p-type doped semiconductor layer. In some infra-redemitting semiconductor materials light can be generated by electronintersub-band transitions rather than electron hole transitions. Herein,the area where the main light generation takes place is termed thelight-emitting layer.

Further, the term “light” is used herein in the sense that it is used inoptical systems to mean not just visible light, but also electromagneticradiation having a wavelength outside that of the visible range.

A major challenge is to extract as much of the emitted light as possiblefrom the semiconductor material into the surrounding medium, typicallyair, thereby increasing the extraction efficiency. This is hindered bytotal internal reflection at the surfaces of the semiconductor. Thischallenge is even greater in the field of μLEDs.

As used herein, the term “extraction efficiency” (EE) encompasses theamount of light extracted from an LED device as a proportion of thetotal light generated by the device. The EE may be expressed as apercentage. Further, the term “μLED” is used herein to encompass an LEDthat is smaller than a standard cuboid LED. A μLED may have an activeregion of approximately 10 μm or greater. Further, a μLED may comprise amesa structure configured to direct light emitted from the active regionto an emission surface in a quasi-collimated fashion.

A common approach to improve the EE of LEDs is to roughen the surfaceswhere the light exits the chip. This reduces the amount of light trappedby total internal reflection that occurs by randomising the angles atwhich the light hits the surface.

It is desirable to increase the EE of LEDs and μLEDs.

SUMMARY

Through experimentation, the inventors have discovered a problem withcurrent LED and μLED devices. As set out herein, crystal structureswithin a device guide the light generated by the active region such thatit reaches the emission surface at an angle greater than or equal to thetotal internal reflection angle. The inventors propose solutionsgenerally to improve the EE of LEDs and μLED and, more specifically, toovercome the problem set out above.

According to one example, there is provided a μLED device comprising: asubstrate and an epitaxial layer grown on the substrate and comprising asemiconductor material, wherein at least a portion of the substrate andthe epitaxial layer define a mesa; an active layer within the mesa andconfigured, on application of an electrical current, to generate lightfor emission through a light emitting surface of the substrate oppositethe mesa, wherein the crystal lattice structure of the substrate and theepitaxial layer is arranged such that a c-plane of the crystal latticestructure is misaligned with respect to the light emitting surface.

Optionally, the substrate and the epitaxial layer comprise asemiconductor material having a wurtzite crystal lattice structure.

Optionally, the semiconductor material comprises Gallium Nitride.

Optionally, the misalignment of the crystal lattice structure issufficient that one or more 1122 planes of the crystal lattice structureis at an angle less than the angle of total internal reflection for theμLED device.

Optionally, the misalignment of the crystal lattice structure is suchthat one or more of the 1122 planes of the crystal lattice structure isat an angle in a range from 0 degrees to 30 degrees from a normal to thelight emitting surface.

Optionally, the c-plane of the crystal lattice structure is misalignedfrom the light emitting surface such that the c-plane is at an angleless than the angle of total internal reflection for the μLED device.

Optionally, the c-plane of the crystal lattice structure is misalignedfrom the light emitting surface by an angle in a range from 85 degreesto 105 degrees.

According to another example, there is provided an array of μLED devicescomprising one or more μLED devices as described above.

Optionally, a plurality of μLED devices in the array are individuallyaddressable.

According to another example, there is provided a method for fabricatinga μLED device comprising: forming a substrate; growing an epitaxiallayer comprising a semiconductor material on the substrate; forming anactive layer from a portion of the epitaxial layer; shaping at least aportion of the substrate and the epitaxial layer into a mesa, whereinthe active layer is within the mesa, the active layer configured, onapplication of an electrical current, to generate light for emissionthrough a light emitting surface of the substrate opposite the mesa,wherein the substrate and the epitaxial layer are arranged such that ac-plane of the crystal lattice structure is misaligned with respect tothe light emitting surface.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments are described herein with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic representation of a section through a μLED device;

FIGS. 2a-c show schematic representations of a wurtzite crystalstructure and the a-plane, c-plane, m-plane and semi-polar plane;

FIG. 3 shows a schematic representation of a wurtzite crystal latticestructure;

FIG. 4 shows a schematic representation of a semiconductor wafer;

FIG. 5 shows an image of light seen at an unpolished light emittingsurface of a μLED device;

FIG. 6 shows an intensity surface contour plot of the light shown inFIG. 5;

FIG. 7 shows a slice through the centre of the intensity data of FIG. 5;

FIG. 8 shows a schematic representation of a section through a μLEDdevice; and

FIG. 9 shows a flow diagram of a method of manufacture of a μLED device.

DESCRIPTION

Generally, disclosed herein are examples of μLEDs and methods ofmanufacture of μLEDs having off-axis crystal structures. The off-axiscrystal structures serve to guide light generated at an active region ofa μLED in directions advantageous to operation of the μLED in a givenapplication. In specific arrangements, the off-axis crystal structuresmay be configured to direct light to be incident on an emission surfaceof the μLED at an angle less than the angle of total internalreflection.

FIG. 1 shows a schematic representation of a μLED 100 comprising asubstrate 102 and an epitaxial layer 104. The substrate 102 and theepitaxial layer 104 may both comprise semiconductor material and mayboth be grown epitaxially using known techniques. The epitaxial layer104 and at least part of the substrate 102 is shaped into a mesastructure 106. An active (or light emitting) layer 108 is enclosed inthe mesa structure 106. The mesa 106 also comprises a further epitaxiallayer 107 that is oppositely doped with respect to the first epitaxiallayer 104. The active layer 108 is positioned between the oppositelydoped semiconductor epitaxial layers 102, 107 and is configured togenerate light when an appropriate electrical current is applied to thedevice 100. The mesa 106 has a truncated top, on a side opposed to alight transmitting or emitting surface 110 of the μLED 100. The lightemitting surface 820 may be a surface of the substrate 102. The mesa 106may also have a near-parabolic shape to form a reflective enclosure forlight generated or detected within the device 100. The arrows 112 showhow light emitted from the active layer 108 is reflected off the wallsof the mesa 106 toward the light emitting surface 110. The lightemitting surface 110 is polished, which creates an angle of totalinternal reflection 111 about a normal to the light emitting surface110. Light incident on the light emitting surface 110 at an angle lessthan the angle of total internal reflection is emitted from the emittingsurface 110. Light incident on the light emitting surface 110 at anangle greater than the angle of total internal reflection is not emittedfrom the emitting surface 110 but is reflected back into the device 100.

The reflection of the light from the internal surfaces of the mesa 106results in light being emitted from μLED 100 in a quasi-collimatedfashion. The term “quasi-collimated” is used herein to define the lightconfined within the critical escape angle of an LED device, e.g. with anangle to a normal less than the angle of total internal reflection.Light generated in the active layer must exit either: (a) directlythrough an exit face without reflection on the mesa sidewall; (b) via asingle reflection on the mesa sidewall resulting in an incident angle tothe exit face within the critical angle range; or (c) following multiplereflections within the mesa structure. Quasi-collimated light has anemission angle lying between the collimated light emitted by a laser andthe uncollimated light emitted by an LED.

A particular exemplary μLED has a wurtzite crystal semiconductorsubstrate and may also have a wurtzite crystal semiconductor epitaxiallayer. In such devices, the individual crystals grown epitaxially formhexagonal tubes or cylinders, as shown in FIGS. 2a and 2b . Theexemplary crystals lattice structures shown in FIGS. 2a and 2b relate tothe crystal lattice structure of Gallium Nitride (GaN).

In FIGS. 2a and 2b , the crystal lattice structures are represented by ahexagonal tube 200, which may be referred to as the unit cell. The termunit cell encompasses the smallest collection of molecules that containsatoms at the vertices of all low index planes of a crystal lattice. Thegallium and nitrogen atoms are shown in FIG. 2. The molecule istetrahedral and comprises four equalateral triangles, which in wurtziteis arranged to produce a hexagonal/cubic symmetry.

Each vertex of each hexagonal ends 201 a, 201 b of the unit cell 200corresponds to the position of an atom in the GaN material. Therefore,the two ends 201 a, 201 b represent adjacent groups of atoms forming ahexagon in the hexagonal crystal lattice structure of GaN. It is notedthat the ends 201 a, 201 b are shown as ends in FIGS. 2a and 2b forexplanatory purposes only. In reality, there may be many more than twoadjacent hexagonally positioned groups of atoms forming a hexagonaltube. A more accurate representation of a GaN wurtzite structure isshown in FIG. 3.

A number of planes of a crystal lattice structure may be defined asconnecting a plurality of atoms within the structure. FIG. 2a shows apolar c-plane 202, a non-polar a-plane 204 and a non-polar m-plane 206.The c-plane 202 is coincident with the plane of the end faces 201 a, 201b of the unit cell 200 and is at right angles to longitudinal axis ofthe unit cell 200. The a-plane 204 is coincident with a plane cuttingthe unit cell 200 between alternate vertices of the hexagonal end faces201 a, 201 b of the unit cell 200. That is, the a-plane 204 iscoincident with a plane that forms a triangle with two outer faces ofthe hexagonal end faces 201 a, 201 b of the unit cell 200. The m-plane206 is orthogonal to the a-plane 204 and the c-plane 202 and iscoincident with an outer longitudinal surface of the hexagonal tube 200.

FIG. 2b shows a higher index, semi-polar plane 208. The semi-polar plane208 may be referred to as the “1122” plane. Further, the family ofsemi-polar planes 208 described below may be defined as the “{1122}”planes. Higher index planes are defined between atoms or molecules ofthe lattice structure that do not correspond longitudinally. That is,the higher order planes run diagonally and longitudinally through thelattice structure.

The higher index, semi-polar plane 208 is coincident with a planerunning diagonally and longitudinally through the unit cell 200. If thevertices of the hexagonal ends of the unit cell 200 where numberedclockwise from 1-6, as in FIG. 2b , the semi-polar plane would runthrough vertices 1 and 3 of a first end (shown by A-B in FIG. 2b ) andthrough vertices 4 and 6 of a second end. The semi-polar plane 208 formsan angle of approximately 32 degrees with the c-plane. It should benoted that there are further higher index planes, which are at the sameangle to the c-plane as the semi-polar plane 208. Such higher indexplanes may pass through points 2 and 4 of the first end 201 a and points1 and 5 of the second end 201 b, points 3 and 5 of the first end 201 aand points 2 and 6 of the second end 201 b, points 4 and 6 of the firstend 201 a and points 3 and 1 of the second end 201 b, points 5 and 1 ofthe first end 201 a and points 4 and 2 of the second end 201 b andpoints 6 and 2 of the first end points 5 and 3 of the second end 201 b.These semi-polar high order planes are not shown in FIG. 2 b.

FIG. 2c shows schematically the a-planes 204 a-f and m-planes 206 a-f ofa wurtzite crystal structure when looking down on the c-plane 202. Itcan be seen that a hexagon is formed by the a-planes 204 a-f. Thishexagon is also defined at the c-plane by the semi-polar planes 208.

FIG. 4 shows an outline of a semiconductor wafer 400 on cm squared graphpaper. Typically, μLED devices are grown on the semiconductor wafer 400having a minor flat 402 coincident with the m-plane 206 of the crystallattice structure and a major flat 404 coincident with the a-plane 204of the crystal lattice structure. Consequently, when the substrate 102and epitaxial layer 104 of a μLED device are grown epitaxially, thehexagonal tubes 200 are aligned vertically on the wafer 400. That is,the c-plane 202 is parallel with the surface of the wafer 400 and, in afinished device, with the light emitting surface 110. This produces ahoneycomb pattern when the crystal lattices are tessellated. In view ofthis, the a-plane 204 and m-plane 206 of each crystal lattice runvertically through the substrate 102 and epitaxial layer 104,substantially parallel to a normal to the light emitting surface 110.

Through experimentation, the inventors have identified a problem withμLED devices having a crystal lattice structure longitudinally alignedwith a normal to the emission surface 110 of the device. That is, theinventors have identified a problem with μLED devices in which thec-plane of the crystal lattice structure is parallel to an emissionsurface of the device.

Using a μLED device 100 with an unpolished light emitting surface 110,the inventors have observed a phenomenon that reduces the EE of μLEDdevices. The phenomenon is shown in FIGS. 5 and 6. FIG. 5 shows an imageof light seen at an unpolished light emitting surface 110 of a μLEDdevice. FIG. 6 shows an intensity surface contour plot of the lightshown in FIG. 5.

The light in FIG. 5 has a wavelength of 405 nm and is emitted from a GaNμLED device having a wurtzite hexagonal crystal lattice structure.However, it is noted that a similar phenomenon has been observed onsimilarly constructed devices having different wavelengths of emittedlight. The light in FIG. 5 can be seen to define a distinct hexagonalemission pattern and has been observed at an unpolished light emittingsurface 110. This hexagonal pattern may be seen more clearly in FIG. 6.The centre-to-edge distance of the hexagonal light emitted is consistentwith the scale of the thickness of a combination of the epitaxial layer104 and the substrate 102. That is, the centre to edge distance of eachhexagon seen at the unpolished light emitting surface 110 corresponds toan angle of 32 degrees inside the epitaxial layer 104 and the substrate102. Specifically, a right angled triangle can be formed from thethickness of the epitaxial layer 104 and substrate 102 combined, thecentre to edge distance of the emitted hexagonal light and the angle atwhich the light is guided through the epitaxial layer 104 and substrate102. Therefore, based on the thickness of the epitaxial layer 104 andsubstrate 102 and the centre to edge distance of the emitted hexagonallight, and angle of approximately 32 degrees is calculated and definesthe hexagon emission at the surface. It is the calculation of 32 degreesthat leads to the conclusion that light is being guided through thecrystal lattice structure along the semi polar plane 208.

The linear horizontal distance shown in FIG. 6, may be converted to apropogation angle inside the substrate and normal to the light emittingsurface 110 by:

${\tan\;\theta} = \frac{w}{h}$

Where θ is the angle of the light propogating inside the substrate tothe normal of the light emitting surface 110, w is the linear horizontaldistance and h is the distance from the active layer 108 to the lightemitting surface 110. Given an exemplary wafer thickness of 350 μm andassuming an exemplary 20 μm diameter μLED can be approximated as a pointsource, the resulting plot of angle of emission against light intensitycan be seen in FIG. 7, which shows a slice through the centre of theintensity data of FIG. 5 with the horizontal distance scale convertedfrom μm to an angle in degrees using the measured wafer parameters andthe equation above. FIG. 7 shows a bright central peak, falling to‘shoulders’ corresponding to the edges of the hexagonal light pattern atapprox 30 degrees from the peak.

The hexagonal pattern has been observed to rotate with the epitaxiallayer and is not considered to be an optical effect of the lenses usedto image the light emitting surface. Near field observation of thesource also does not indicate any hexagonal structure. The inventorshave concluded that the observed pattern of the intermediate radiantfield in the crystal is a previously unobserved property of the wurtzitecrystal caused by emitted light being guided along the semi-polar plane208, defined above.

The polar c-plane 202 and the semi-polar plane 208 have lower symmetryand their electronic and optical properties are direction dependent dueto this asymmetry and anisotropic strain components causingelectron-hole polarization. The refractive index in the c-plane 202 andthe semi-polar plane 208 are also affected and photons travelling in ornear these planes will be index-guided in much the same way as light isindex guided in an optical fibre.

The inventors have identified that this effect is causing the hexagonallight pattern at the unpolished light emitting surface 110 of the μLEDdevice. This is because the hexagonal light pattern is observed at anangle of approximately 32 degrees, which corresponds to the angle of thesemi-polar plane 208 to a normal to the c-plane 202 identified in FIG.2b . In typical devices, the c-plane 202 is coincident with the lightemitting surface 110, so the angle of 32 degrees is also with respect toa normal to the light emitting surface 110. The other planes in thedirection of propogation (a and m) are non-polar and therefore notlikely to contribute to index guiding.

In exemplary μLED devices 100, the refractive index of the substrate 104may be approximately 2.5. In such devices, when the light emittingsurface 110 is polished, the resulting angle of total internalreflection is approximately 23.5 degrees. The hexagonal light patternshown in FIGS. 5 and 6 is emitted at an angle lying outside the angle oftotal internal reflection and is therefore internally scattered and notnormally observed. This internal scattering represents a significantradiant emission loss mechanism for any μLED structure, reducing the EE,impacting directly both external quantum efficiency and total radiantpower. Also in addressable μLED device arrays it would increasecross-talk between μLED devices in the array, as the internallyscattered light would re-scatter from any feature it subsequentlyencountered and contribute to noise in adjacent μLED devices.

The capture of this emission via the control or suppression of thiseffect will therefore increase EE per pixel and reduce addressabledevice cross-talk. Also utilising the effect could yield novel devicesas yet unknown.

FIG. 8 shows a schematic representation of an exemplary μLED device 800that is not drawn to scale. The device 800 comprises a substrate 802 andan epitaxial layer 804, grown on the substrate and a mesa 806 formed atleast partly from the substrate 802 and the epitaxial layer 804. Thesubstrate 802 is typically one or two orders of magnitude thicker thanthe epitaxial layer 804. Typically, the thickness of the epitaxial layer804 is in a range from 1-5 μm and the substrate is in the order of a fewhundred μm. An active layer 808 is within the mesa 806 and is configuredto generate light on application of a given electrical bias. Lightgenerated from the active layer 808 is emitted from the μLED device 800through a light emitting surface 810, as described above in relation toFIG. 1. The light emitting surface 810 may be polished and, therefore,only light incident on the light emitting surface 810 within a givenangle 811 to a normal 812 to the light emitting surface 810 is emitted.The given angle 811 may be termed the angle of total internalreflection. Light incident on the light emitting surface 810 at an anglegreater than the angle of total internal reflection is reflected backoff the light emitting surface 810 into the μLED device 800. Insulationand metallisation layers are not shown in FIG. 8, although they may beadded to practical implementations of the μLED 800.

The substrate 802 of the μLED device 800 comprises a crystal latticestructure. In the exemplary μLED device 800, the crystal latticestructure is a wurtzite structure and the epitaxial layer 804 comprisesGaN. The substrate 802 has been formed such that a longitudinal axis ofthe crystal lattice structures is misaligned with respect to a normal812 to the light emitting surface 810. In exemplary μLED devices, thesubstrate 802 has been grown such that the c-plane 202 of the crystallattice structures is similarly misaligned with respect to the lightemitting surface 810. The epitaxial layer 804 is grown on top of thesubstrate 802 and so the crystal lattice structure of the epitaxiallayer 804 has the same orientation as the crystal lattice structure ofthe substrate 802.

FIG. 8 shows schematically an exemplary misalignment of the crystalstructure of the wurtzite crystal lattices 814 a-c that make up thesubstrate 802. It is noted that the wurtzite crystal lattices 814 a-care shown schematically only for the purpose of easier explanation andneed not be considered to scale. In addition, the exemplary misalignmentof the crystal lattice structure is for explanatory purposes and is notnecessarily indicative of the misalignments used in the invention.

Considering an exemplary crystal lattice 814 a, a longitudinal axis 816runs along the length of the lattice 814 a. The c-plane 202 of thecrystal lattice 814 a is perpendicular to the longitudinal axis 816. Alongitudinal side 818 of the crystal lattice 814 a is perpendicular tothe longitudinal axis 816 and may be coincident with either of thea-plane 204 or the m-plane 206 (not shown in FIG. 8). The semi-polarplane 208 of the crystal lattice 814 a is also shown in FIG. 8. Thesemi-polar plane 208 is a plane of the crystal lattice 814 a that is atan angle to the longitudinal axis 816 in a range of 30 degrees to 35degrees, more specifically, approximately 32 degrees.

The c-plane 202 of the crystal lattice 814 a is misaligned with respectto the light emitting surface 810 in that it is not parallel thereto.The c-plane 202 is at an angle 820 with respect to the light emittingsurface 810. It can also be seen that the longitudinal axis 816 of thecrystal lattice 814 a is at an angle to the normal 812 that is equal tothe angle 820. In exemplary μLED devices, the angle 820 may besufficient to ensure that the semi-polar plane 208 lies at an angle tothe normal 812 that is less than the angle of total internal reflection811. This can be seen in FIG. 8, as the arrow representing thesemi-polar plane 208 is within the angle of total internal reflection811. In further exemplary μLED devices, the angle 820 may be in a rangefrom 2 degrees to 50 degrees, in a range from 5 degrees and 50 degreesor in a range from 5 degrees to 10 degrees. In further exemplary μLEDdevices, the semi-polar plane 208 may be at an angle to the normal 812in a range from 0 degrees to 30 degrees.

By aligning the semi-polar plane 208 such that it is within the angle oftotal internal reflection, the μLED device is able to solve or mitigateone or more of the problems disclosed herein. Specifically, lightgenerated by the active layer 808 and guided by the semi-polar plane 208is incident on the light emitting surface 810 of the μLED device 800 atan angle less than or equal to the angle of total internal reflection811.

In yet further exemplary μLED devices, the crystal lattice structure ofthe substrate 802 and epitaxial layer 804 may be configured such thatlight emitted from the active layer 808 is guided by the c-plane 202such that it is incident on the light emitting surface 810 at an angleless than the angle of total internal reflection. In exemplary devices,the c-plane 202 may be in a range from 0 degrees to 30 degrees to thenormal 812 to the light emitting surface 810. In a specific device, thec-plane 202 may be substantially coincident with the normal 812.

A method of fabricating an example of a μLED device 800 is describedwith reference to FIG. 9. A semiconductor substrate 802 is grown 900such that the c-plane 202 of the crystal lattice structure of thesemiconductor material is misaligned with respect to the light emittingsurface 810 of the μLED device 800, as described above.

First and second epitaxial layers 804, 807 are grown 902. The firstepitaxial layer 804 is epitaxially grown on the substrate 802 and thefurther epitaxial layer 807, oppositely doped, is grown on the firstepitaxial layer 804.

An active layer 808 is formed 904 between the first and furtherepitaxial layers 804, 807. The active layer 808 is configured togenerate light when an electrical current is applied to the deviceand/or to generate an electrical current when light is incident on theactive layer 808.

A surface of the further epitaxial layer 807 is shaped 904 to form amesa 806 comprising at least part of the further epitaxial layer 807, atleast part of the active layer 808 and at least part of the firstepitaxial layer 804. This may be done by etching and methods suitableare set out in WO 2004/097947.

For clarity, many of the steps required to manufacture a complete μLEDdevice are not shown in FIG. 9, although these will be known to theskilled person. Similarly, many further features will be required inFIG. 8 to make the device 800 operational, but these are not shown, forclarity purposes.

Whilst specific embodiments are described herein, it will be appreciatedthat a number of modifications and alterations may be made theretowithout departing from the scope of the disclosure, as set out in theappended claims.

What is claimed is:
 1. A μLED device comprising: a substrate and anepitaxial layer grown on the substrate and comprising a semiconductormaterial, wherein at least a portion of the substrate and the epitaxiallayer define a mesa; an active layer within the mesa and configured, onapplication of an electrical current, to generate light for emissionthrough a light emitting surface of the substrate opposite the mesa,wherein a crystal lattice structure of the substrate and the epitaxiallayer is arranged such that a c-plane of the crystal lattice structureis misaligned with respect to the light emitting surface, such that oneor more 1122 planes of the crystal lattice structure is at an angle froma normal to the light emitting surface which is less than an angle oftotal internal reflection for the μLED device.
 2. A μLED deviceaccording to claim 1, wherein the substrate and the epitaxial layercomprise a semiconductor material having a wurtzite crystal latticestructure.
 3. A μLED device according to claim 1, wherein thesemiconductor material comprises Gallium Nitride.
 4. A μLED deviceaccording to claim 2, wherein the semiconductor material comprisesGallium Nitride.
 5. A μLED device according to claim 1, wherein themisalignment of the crystal lattice structure is such that one or moreof the 1122 planes of the crystal lattice structure is at an angle in arange from 0 degrees to 30 degrees from a normal to the light emittingsurface.
 6. A μLED device according to claim 2, wherein the misalignmentof the crystal lattice structure is such that one or more of the 1122planes of the crystal lattice structure is at an angle in a range from 0degrees to 30 degrees from a normal to the light emitting surface.
 7. AμLED device according to claim 1, wherein the c-plane of the crystallattice structure is misaligned from the light emitting surface suchthat the c-plane is at an angle less than the angle of total internalreflection for the μLED device.
 8. A μLED device according to claim 2,wherein the c-plane of the crystal lattice structure is misaligned fromthe light emitting surface such that the c-plane is at an angle lessthan the angle of total internal reflection for the μLED device.
 9. AμLED device according to claim 1, wherein the c-plane of the crystallattice structure is misaligned from the light emitting surface suchthat the c-plane is at an angle less than the angle of total internalreflection for the μLED device.
 10. A μLED device according to claim 1,wherein the c-plane of the crystal lattice structure is misaligned fromthe light emitting surface by an angle in a range from 85 degrees to 105degrees.
 11. A μLED device according to claim 2, wherein the c-plane ofthe crystal lattice structure is misaligned from the light emittingsurface by an angle in a range from 85 degrees to 105 degrees.
 12. AμLED device according to claim 1, wherein the c-plane of the crystallattice structure is misaligned from the light emitting surface by anangle in a range from 85 degrees to 105 degrees.
 13. An array of μLEDdevices, each μLED device comprising: a substrate and an epitaxial layergrown on the substrate and comprising a semiconductor material, whereinat least a portion of the substrate and the epitaxial layer define amesa; an active layer within the mesa and configured, on application ofan electrical current, to generate light for emission through a lightemitting surface of the substrate opposite the mesa, wherein a crystallattice structure of the substrate and the epitaxial layer is arrangedsuch that a c-plane of the crystal lattice structure is misaligned withrespect to the light emitting surface, such that one or more 1122 planesof the crystal lattice structure is at an angle from a normal to thelight emitting surface which is less than an angle of total internalreflection for the μLED device.
 14. The array of μLED devices accordingto claim 13, wherein a plurality of μLED devices in the array areindividually addressable.
 15. A method for fabricating a μLED devicecomprising: forming a substrate; growing an epitaxial layer comprising asemiconductor material on the substrate; forming an active layer from aportion of the epitaxial layer; shaping at least a portion of thesubstrate and the epitaxial layer into a mesa, wherein the active layeris within the mesa, the active layer configured, on application of anelectrical current, to generate light for emission through a lightemitting surface of the substrate opposite the mesa, wherein thesubstrate and the epitaxial layer are arranged such that a c-plane of acrystal lattice structure is misaligned with respect to the lightemitting surface, such that one or more 1122 planes of the crystallattice structure is at an angle from a normal to the light emittingsurface which is less than an angle of total internal reflection for theμLED device.